I Can Guess the Data Structure!

There is a bag-like data structure, supporting two
operations:

$1\ x$: Throw an
element $x$ into the
bag.

$2$: Take out an
element from the bag.

Given a sequence of operations with return values, you’re
going to guess the data structure. It is a stack (Last-In,
First-Out), a queue (First-In, First-Out), a priority-queue
(Always take out larger elements first) or something else that
you can hardly imagine!

Input

There are several test cases. Each test case begins with a
line containing a single integer $n$ ($1\leq n\leq 1000$). Each of the next
$n$ lines is either a
type-1 command, or an integer $2$ followed by an integer
$x$. This means that
executing the type-2 command returned the element $x$. The value of $x$ is always a positive integer not
larger than $100$. The
input is terminated by end-of-file (EOF). The size of input
file does not exceed 1MB.

Output

For each test case, output one of the following:

stack
It’s definitely a stack.

queue
It’s definitely a queue.

priority queue
It’s definitely a priority queue.

impossible
It can’t be a stack, a queue or a priority queue.

not sure
It can be more than one of the three data structures mentioned
above.